Dense Wavelength Division Multiplexing (hereinafter, “DWDM”) Technology, which employs a single optical fiber with optical signals each having different wavelength in place of a plurality of optical fibers, have proceeded over the past few years. In the systems that employ the DWDM technology (hereinafter, “DWDM systems”), one wavelength is assigned a band of a few gigabits (Gbits), and the single optical fiber makes it possible to communicate through a few dozens of wavelengths (i.e., a few hundred gigabits). The DWDM systems require higher stability with narrow spacing between the wavelength bands, to multiplex the optical signals in each of different wavelength bands in high density. An optical communication device applied to the DWDM systems employs a semiconductor module that outputs an optical signal having a specific wavelength band from a laser diode (hereinafter, “semiconductor laser”). The wavelength division multiplexing on the DWDM systems is realized by multiplexing beams, which are output from a plurality of semiconductor modules each having different emission wavelength bands, in one optical fiber.
This type of semiconductor module is disclosed in, for example, Japanese Patent Application Laid-open No. 2001-244557. The application, specifically, discloses, monitoring a wavelength of a beam emitted from the rear side of the semiconductor laser, adjusting a current injected to the semiconductor laser and a temperature to stabilize an emission wavelength of the semiconductor laser within a specific wavelength band, and thereby controlling the emission wavelength.
It is generally known that the emission wavelength of the semiconductor laser increases with an increase in the injection current for high beam power. It is also known that the wavelength changes depending on a temperature, and the emission wavelength increases with an increase in the temperature. The application also discloses a technique of monitoring an emission wavelength of a semiconductor laser using a birefringent crystal that has an optical anisotropy in which refractive indexes thereof differ depending on polarization directions of an input laser beam. The birefringent crystal is provided with a high reflection coating on both end surfaces thereof. In the application, moreover, the use of two crystals to cancel a change of a refractive index depending on a temperature of the birefringent crystal is disclosed. One crystal is a YVO4 crystal whose refractive index increases with an increase in a temperature thereof, and the other crystal is a β-BaB2O4 crystal whose refractive index decreases with an increase in a temperature thereof.
“Temperature Compensation of Birefringent Optical Filter, Kimura et al, the proceedings of IEEE, August 1971, pp. 1273-1274” also discloses an optical filter that employs two birefringent crystals. FIG. 1 shows a structure of this optical filter. The optical filter consists of a first polarizer 51, a CaCO3 crystal 52, an LiTaO3 crystal 53, and a second polarizer 54.
The first polarizer 51 transmits only a linearly polarized beam. The linearly polarized beam is inclined by 45 degrees from a horizontal axis direction on a plane that is perpendicular to an incident light axis direction. The CaCO3 crystal 52 has a fast axis in a plane that is perpendicular to an incident light. The LiTaO3 crystal 53 has a slow axis in a plane that is perpendicular to the incident light. The second polarizer 54 transmits only a linearly polarized beam in a polarization direction that is the same as the first polarizer. The fast axis refers to an axis direction in which a refractive index is the lowest on the plane that is perpendicular to an optical axis of the birefringent crystal. The slow axis refers to an axis that is perpendicular to the fast axis.
As a result, a laser beam is incident on the first polarizer 51. The laser beam that has passed through the first polarizer 51 has only the component inclined by 45 degrees from the horizontal axis direction on the plane that is perpendicular to the optical axis direction. Therefore, the polarized beam inclined by 45 degrees with respect to the crystal axes of the CaCO3 crystal 52 and the LiTaO3 crystal 53 is sequentially incident on the crystal. The CaCO3 crystal 52 and the LiTaO3 crystal 53 give different phase shifts to the incident laser beam that pass through these crystals. The second polarizer 54 transmits only the polarization component that is the same as the polarized beam that has been transmitted from the first polarizer 51. Since the phase shifts given in the crystals 52 and 53 have wavelength dependency, the intensity of the laser beam that has passed through the second polarizer 54 also has wavelength dependency.
However, the wavelength of the laser beam that has passed through the second polarizer 54 is not easily changed by a temperature change. This is because the CaCO3 crystal 52 and the LiTaO3 crystal 53 have such lengths l1 and l2 respectively that respective phase shift quantities δ are cancelled each other with respect to a temperature change. Concretely, the shift quantities and the shift directions of the length l1 and l2 are designed.
The LiTaO3 crystal 53 is a birefringent crystal material that can be manufactured in a large quantity at low cost. The inventors have previously proposed, in Published International Application WO 01/5787A1, a method of monitoring an emission wavelength of a semiconductor laser, by using an LiNbO3 crystal that is a birefringent crystal material that can be manufactured in a large quantity at low cost, like the LiTaO3 crystal 53. The proposed method describes that a wavelength filter is structured using the LiNbO3 crystal having typical two characteristics. One characteristic is that refractive indexes of the LiNbO3 crystal are different depending on an axis azimuth based on the optical anisotropy of the LiNbO3 crystal. The other characteristic is that a polarization state of a laser beam that has passed through the LiNbO3 crystal changes depending on the wavelength of the laser beam. As a result, this method makes it possible to monitor the emission wavelength of the semiconductor laser by detecting the polarization state of the laser beam that has passed through the LiNbO3 crystal as the birefringent crystal. In the proposed method, moreover, a combination of two crystals is employed. One crystal is the YVO4 crystal whose refractive index increases with an increase in a temperature thereof. The other crystal is the LiNbO3 crystal whose refractive index decreases along an increase in a crystal temperature. As a result, the combination makes it possible to cancel a change of a refractive index relative to a temperature change of those birefringent crystals.
The inventors, however, have found the following problems of the proposed method by an experiment. The wavelength filter has the combination so as to cancel the change of the refractive index relative to a temperature change. However, a phase shift relative to the temperature change remains depending on a birefringent crystal material that is employed in the combination. This remaining phase shift gives an influence that cannot be disregarded to the monitoring of a wavelength.
In other words, even when a laser beam that is stably emitted with a wavelength (reference wavelength) to be controlled is incident on the wavelength filter, the wavelength of the beam after passing through the wavelength filter varies due to a change in the temperature. In this experiment, the inventors have used the LiNbO3 crystal as one of the birefringent crystals that constitute the wavelength filter. The thickness of the LiNbO3 crystal in a propagation direction of the laser beam is extremely small as compared with the size of the plane on which the laser beam is incident. However, pyroelectricity of the LiNbO3 crystal leads to a change of the phase shift quantity δ, and therefore the wavelength discrimination characteristic of the wavelength filter changes from a temperature change and an external stress. It is not possible to disregard this change in monitoring the emission wavelength of the semiconductor laser in high precision. Moreover, the pyroelectricity causes piezoelectric effect, and this piezoelectric effect further leads to an increase in the change of the phase shift quantity δ.
Japanese Patent Application Laid-open No. 62-73207 discloses an example of the application of the LiNbO3 crystal to an optical waveguide device. A change in temperature of the optical waveguide leads to the result that pyroelectricity of a substrate made of the LiNbO3 crystal changes a state of electric polarization. Therefore, a charge is accumulated on the surface of the substrate. Corresponding to the accumulated charge, the opposite polarity of charge is induced on the bottom surface of an electrode. A refractive index of the waveguide is changed by applying an electric field to between the electrode and the counter electrode. A modulation operation of the optical waveguide device is realized by this changes in the refractive index.
However, when a charge occurs within the waveguide due to a change in the temperature, modulation characteristics change significantly over the waveguide length. The Japanese Patent Application describes that it is possible to suppress the change in the modulation characteristics resulting from the temperature change, in the following manner. An ITO (Indium Tin Oxide) film is coated on the surface of the substrate over the waveguide length so that charge distribution on the surfaces of the electrodes and the surface between the electrodes is uniform. This uniform charge distribution prevents the generation of an electric field that is directed from between the electrodes to one electrode, and suppresses a change in the modulation characteristics resulting from the temperature change.
However, the charge accumulated on the surface of the LiNbO3 substrate is not localized but is accumulated uniformly. Therefore, refractive index variation occurs due to the electro-optic effect. While the ITO film induces a charge on the surface thereof, this substance is relatively expensive in general, and it has been relatively difficult to handle the coating and the like.
As other birefringent crystal material, an SiO2 crystal is generally used, as described in a brochure of U.S. VLOC Incorporated, titled “LASER OPTICS, COATINGS, CRYSTALS AND CAVIIES”, pp. 9-13 (issued by VLOC). This SiO2 crystal is obtained only as a natural substance, and is expensive. Therefore, it is difficult to produce a large number of the SiO2 crystal.
The conventional wavelength filter is structured as explained above. While two birefringent crystals are employed to cancel a change in the refractive index resulting from a temperature change, pyroelectric materials are used for the birefringent crystals. Therefore, the pyroelectric effect occurs, and the piezoelectric effect occurs following the pyroelectric effect. Consequently, there is a problem that these effects give an influence that cannot be disregarded to improving the precision of the wavelength discrimination characteristics.
According to the conventional optical waveguide device that employs the pyroelectric material, the ITO film is coated on the surface on which the electrode is disposed, and therefore a charge does not exist on the surface locally. However, a charge is uniformly accumulated on the surface of the substrate, which also generates a change in the refractive index. While the ITO film is employed to induce a charge on the surface, ITO is expensive in general, and it is relatively difficult to handle the coating and the like.
To solve the problems, it is an object of the present invention to provide a wave plate and a wavelength filter that employ a birefringent crystal, such as the LiNbO3 crystal or the LiTaO3 crystal that can be manufactured in a large quantity at low cost, that suppress the pyroelectric effect and the piezoelectric effect of these crystals, and that have polarization characteristics and wavelength discrimination characteristics that are stable against changes in the environment such as temperature and external stress. It is another object of this invention to provide a wavelength monitor that employs the wave plate or the wavelength filter.